Sydney's driverless Metro line opens

Some 42,000 commuters have flocked to Sydney's new driverless northwest Metro in its first three hours of operation after it was officially opened on Sunday morning.

The first people boarded trains at Tallawong Station in Rouse Hill following the opening by Premier Gladys Berejiklian and Transport Minister Andrew Constance.

"This is the start of something very special for our state," Mr Constance told reporters.

"Life's about to change ... for so many people who work hard every day and deserve the right to be able to get around the city as easily as they can."

Mr Constance said he was looking forward to Monday "with the first commute in the morning peak".

The fully-automated trains are required to operate with a 98 per cent on-time running reliability, a government fact sheet says.

Sydney Metro asked for patience on Twitter while announcing the service had seen 42,000 customers in the first three hours of operation.

Commuters took to Twitter to report large crowds at Chatswood Station, trains stuck at Macquarie University and Macquarie Park stations, as well as service gaps at numerous stations.

The network experienced a hiccup when a technical problem with doors on one train caused a service gap of between 15 and 20 minutes. The train was taken out of service to be looked at, Sydney Metro tweeted.

More than 20,000 people worked on the $7.3 billion northwest project over eight years and work is now underway to extend the line from Chatswood to Bankstown by

"We're now into the complexities of building under the Harbour, under the sea, and out to Sydenham and on to Bankstown," Mr Constance said.

Later, another metro project will link greater Parramatta with the CBD.

When the newly-opened northwest line reaches full operation, trains will run every four minutes each way during peak hour between Tallawong and Chatswood.

But they will first run once every five minutes at peak for about six weeks, which Mr Constance previously described as a "ramp-up period".

Ms Berejiklian said she couldn't wait for people to try the service and give their verdict.